To meet the demands of the industry, it has generally become desirable to manufacture semiconductor devices having features that are continually decreasing in size. This requirement has led to the development of lithographic processes, capable of increased accuracy, to produce such features. In practice, however, such processes have been found to be correspondingly difficult to implement.
For example, the implementation of lithographic techniques such as 248-nm lithography have permitted the manufacture of features of a size sufficiently small to meet many practical demands. The extension of 248-nm lithography to such applications has in many cases necessitated the introduction of correspondingly more sensitive photoresist materials, however, such as the "deep ultraviolet" (DUV) photoresists. Such photoresist materials have tended to demonstrate poor performance with traditional etching processes, exhibiting a significant potential for damage. This potential for damage to the photoresist has typically been addressed by the application of less aggressive etching processes (i.e., using lower power, less reactant flow, etc.). Such measures lead, in turn, to a loss of etch rate, and possibly a reduction in anisotropy.
It has been found that, in many cases, a "high density plasma" (HDP) process can be used to improve throughput and to better control critical dimension (CD). High-density plasma sources typically offer a much higher etch rate than traditional, diode-type reactors. This higher rate results because the plasma electrons are excited in a direction normal to the reactor boundaries, which allows the operating pressure to be reduced to a point where the electron mean free path is much larger than the physical size of the reactor. In order to be self-sustaining, the plasma attains a higher electron and reactive neutral density, to compensate for greater diffusion losses.
It has further been found, however, that the accuracy of a lithographic process can be improved using an "organic anti-reflective coating" (ARC) to improve the apparent flatness of the reflective surfaces established in the photoresist materials. Although they facilitate the development of more precise (and smaller) features, such anti-reflective coatings tend to have etching characteristics similar to the photoresist materials with which they are used. This similarity makes it difficult to avoid damage to the photoresist material while at the same time ensuring that the anti-reflective coating is effectively opened.
As a result, when applied to ARC etching applications, high density plasma etching processes (or "tools") have been found to suffer from poor DUV photoresist protection due to their aggressiveness (i.e., high ion flux, high dissociation fraction, etc.). This drawback has tended to complicate the implementation of desirable characteristics (features) using such high-density plasma sources. For this reason, ARC openings for sensitive photoresist materials have generally been formed using a traditional process tool to minimize the potential for damage, presumably by limiting the etch rate and the reactant flow. Much of the throughput advantage of a high-density plasma tool is lost, however, when a conventional tool must be used to open the ARC layer.
Previous state-of-the-art tools using high-density plasma to open an anti-reflective coating have relied on the addition of oxidants to combust the organic material. For example, mixtures of He and O.sub.2 using very low power levels (e.g., a 150 Watt bias or less) have shown promise for less sensitive photoresist materials, such as polyhydroxystyrene resists having protecting groups pendant from some of the styrene groups. Similarly, mixtures of Ar and CO.sub.2, using low bias powers, have also performed adequately on such materials. Both of these mixtures have shown unsatisfactory microfissures, however, on the more sensitive photoresist materials, such as resists containing a combination of hydroxystyrene and protecting acrylate monomers in the same polymer chain, as is shown in FIG. 1. In addition, with more sensitive photoresist materials, the photoresist can be laterally attacked by oxidants, leading to a loss of critical dimension (CD) control, as is shown in FIG. 2.
One approach to decrease both microfissures and CD growth is to include a polymerizing agent in the ARC etch. The deposition of the polymer requires an ion sputtering component for the etching to proceed, which increases the anisotropy of the etch process. In addition, the deposited polymer can reduce the degree of microfissures due to selective deposition.
As an example, the effect of adding a polymer to an oxidant-based ARC etch is shown in FIG. 3. It has been found that, in practice, the bias power must be increased to overcome the polymer deposition. Once again, while the resulting performance is satisfactory on less sensitive photoresist materials, damage still tends to occur for the more sensitive photoresist materials.
Therefore, a primary object of the present invention is to provide an improved etching system for opening an organic anti-reflective coating. Another object of the present invention is to provide an improved etching system for opening an organic anti-reflective coating using a high-density plasma source. Still another object of the present invention is to provide an improved etching system for opening an organic anti-reflective coating, using a high-density plasma source, while minimizing damage to the photoresist material.